Condition variables are used when one thread wants to wait until another
thread has finished doing something: the former thread ``waits'' on the
condition variable, the latter thread ``signals'' the condition when it
is done. Condition variables should always be protected by a mutex.
The typical use is (if D is a shared data structure, m its mutex,
and c is a condition variable):

wait c m atomically unlocks the mutex m and suspends the
calling process on the condition variable c. The process will
restart after the condition variable c has been signalled.
The mutex m is locked again before wait returns.